To assess the implications of detection of interleukin 8 (IL-8) in urine.IL-8 was measured by immunoassay in all 305 urine samples from children aged 0-18.4 years received by our microbiology laboratory during four weeks, with a retrospective structured case note audit for all those in whom IL-8, white cells, or bacteria were detected. Patients were divided into three groups: urinary tract infection (UTI), at least one sample with >/=5 leucocytes x 10(9)/l and >/=10(5) cultured bacteria/ml; possible UTI, at least one sample with >/=5 leucocytes x 10(9)/l or >/=10(5) cultured bacteria/ml but not both; UTI unlikely, sample(s) with <5 leucocytes x 10(9)/l and <10(5) cultured bacteria/ml. Medical records were sought for all in groups 1 (14/14 found) and 2 (18/21 found) and those in group 3 (41/59 found) in whose urine any leucocytes, cultured bacteria, or IL-8 were detected.IL-8 was detected in 58/305 samples from 48/264 patients. IL-8 was detected in at least one urine sample from 13/14 patients with confirmed UTI (group 1); in 11/21 patients with possible UTI (group 2), of whom two were treated as UTI; and in 23/228 patients without UTI. Using a cut off of 200 pg/ml, urine IL-8 had a sensitivity of 93% and a specificity of 90% for diagnosing UTI.Urine IL-8 is a sensitive test for UTI, but is poorly specific as it is also present in a variety of other infectious and inflammatory disorders.